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© Matthias Liepe, 2012
Recap Lecture 19
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Today:
• Magnetic materials
• Change in magnetic flux and Faraday’s law of induction
• Lenz’s law
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Magnetic Materials
• Ferromagnetic: • Examples: Iron, nickel… • Divided into regions (“domains”) in
which atomic magnetic dipoles line up • If placed in an external magnetic field:
dipoles of domains line up in direction of magnetic field -> material develops a strong magnetic
dipole moment in direction of the applied external magnetic field
• The dipole moment alignment (“magnetization”) partially persists when the external field is removed -> permanent magnet
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That’s why a magnet sticks to a steel refrigerator door…
S
N
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Example: Hard disk drive
A hard disk drive records data by magnetizing a thin film of ferromagnetic material on a disk.
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• Diamagnetic: • Atoms have no permanent dipole moments, but weak magnetic
dipole moments are produced in the atoms when placed in an external magnetic field
->Create a very weak magnetic dipole field in opposition to an externally applied magnetic field
• Dipole moments and net field disappear when external magnetic field is removed
• Paramagnetic: • Atoms have permanent dipole moments, but are randomly
oriented • When placed in an external magnetic field, dipoles partially align in
direction of the field ->Create a net magnetic dipole field in direction of the
externally applied magnetic field • Alignment and net field disappear when external magnetic field is
removed
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Example: Levitation of a Frog on a strong Magnetic Field
• A live frog levitates inside a 32 mm diameter vertical bore of a solenoid in a magnetic field of about 16 T.
• Why? • Diamagnetism
of the frog • Magnetic dipole
moment of frog opposes Bext -> repulsive force!
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